The present invention relates to a control system for a disk array which has a plurality of disk units arranged in an array form used as an external storage device for a computer system, and more particularly to a control system for optimizing parity data update processing.
As a disk system comprising a plurality of disk units, Edward K. Lee and Randy H. Katz of University of California at Barkeley have proposed, in an article, an architecture of a disk system which is provided with a parallel interface for each of disk units so as to allow the disk units to simultaneously send a plurality of I/O requests to the interfaces, thus achieving a fast-access and large scale disk system.
Also, for a disk unit access system at "level 4, 5" according to the classification of disk arrays into five levels, it has been known, for example, that parallel read accesses are achieved at level 4, whereas, since a write access requires read modify write processing (RMW processing) to be performed on previous data including parity data, the level 4, which assigns parity data to a single disk unit, limits only one write access request from a host computer to a disk array". Thus, "level 4, 5" for controlling accesses to disk units in striping size units has a problem that the RMW processing should be executed the fastest possible. According to the above-mentioned article published from the University of California at Barkeley entitled "Performance Consequences of Parity Placement in Disk Arrays", two systems are proposed for realizing a parity data update processing system (RMW processing system).
However, since the above-mentioned prior art requires the data read/write processing for reading data from and writing data into disk units when parity data is to be updated, the performance of the whole system is limited by the disk access capability.
Further, U.S. application Ser. No. 07/827,982 corresponding to JP-A-4-245352 discloses a storage unit control method and a storage unit sub-system for speeding-up of write processing to an array type disk system.